1. Technical Field of the Invention
The present invention relates to voltage control apparatuses for controlling output voltages of electric generators that are used in motor vehicles such as passenger cars and trucks.
2. Description of the Related Art
In recent years, to reduce emissions from motor vehicles and improve the fuel economy of the same, both engine idle speed and engine friction have been lowered. As a result, a variation in the torque of an automotive electric generator, which is driven by the engine of a motor vehicle, may cause hunting of the engine to occur in some rotational speed region. Hereinafter, hunting of the engine denotes a phenomenon in which the rotational speed of the engine fluctuates and thus cannot keep constant.
To prevent hunting of the engine from occurring due to a variation in the torque of an automotive electric generator, Japanese Unexamined Patent Application Publication No. 2002-17053 (to be referred to as Patent Document 1 hereinafter) discloses an approach. According to this approach, the generator is controlled by setting a target voltage of the generator according the rotational speed of the generator. Consequently, it becomes possible to prevent hunting of the engine from occurring in a rotational speed region in which the torque of the generator suddenly changes from increasing to decreasing or vice versa over the torque peak of the generator. Hereinafter, the torque peak denotes an operating point of the generator at which the torque of the generator reaches its peak.
However, with the aging of an automotive battery that is charged by the generator, the drop in the terminal voltage of the battery clue to the application of an electrical load may become large. Therefore, with the approach disclosed in Patent Document 1, it may be difficult for the rotational speed of the engine to follow the large drop in the terminal voltage of the battery. As a result, the large drop in the terminal voltage of the battery may trigger hunting of the engine so that the rotational speed of the engine fluctuates and gradually converges on a target idle speed.
Moreover, with the approach disclosed in Patent Document 1, since the target voltage of the generator is set according to the rotational speed of the generator, the target voltage also fluctuates in sync with the rotational speed of the engine. Consequently, the charging voltage of the battery also fluctuates, influencing the brightness of lighting and display devices on the vehicle and thereby making the driver of the vehicle uncomfortable.
In addition, Patent Document 1 also introduces another approach that is originally disclosed in Japanese Unexamined Patent Application Publication No. H7-123796 (to be referred to as Patent Document 2 hereinafter). According to this approach, upon detection of the application of an electrical, load, the target voltage of the generator is lowered once and then gradually returned to its original value.
However, with the approach disclosed in Patent Document 2, it is difficult to detect the application of an electrical load without a delay only when the engine is idling. Moreover, it is also difficult to suppress the fluctuation of the rotational speed of the engine during idling of the engine only by means of a voltage control apparatus that regulates the output voltage of the generator to the target voltage.
Japanese Unexamined Patent Application Publication No. 2007-295659 (to be referred to as Patent Document 3 hereinafter) discloses still another approach for suppressing fluctuation of the rotational speed of an engine during its idling in a rotational speed region higher than the torque peak speed of the engine. Hereinafter, the torque peak speed of the engine denotes the rotational speed of the engine when the torque of an electric generator driven by the engine is at its peak. Similarly, the torque peak speed of an electric generator denotes the rotational speed of the generator when the torque of the generator is at its peak.
More specifically, according to the approach disclosed in Patent Document 3, when a decrease in the rotational speed of the generator is detected, a field current control is performed to limit field current supplied to the generator. Consequently, the decrease in the rotational speed is suppressed, thereby suppressing the drop in the terminal voltage of an automotive battery that is charged by the generator. On the other hand, when no decrease in the rotational speed of the generator is detected, the field current control is disabled, thereby allowing the output voltage of the generator to be regulated to a target voltage. As a result, with this approach, it is possible to stabilize both the idle rotation of the engine and the terminal voltage of the battery.
However, with the approach disclosed in Patent Document 3, it may be difficult to achieve the above-described effects when the rotational speed of the generator decreases below the torque peak speed of the generator.